MONGODB 3.0BIG POLYGON

Dave Erickson

Equator

Prime MeridianAntimeridian

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But what’s the inside of the polygon?

Define a polygon by specifying 4 points

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Convention for deciding “inside”:Winding Order + Right Hand Rule

Positive = Inside of Polygon

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But how does MongoDB pick the inside of the Polygon?

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Mongo 2.6 behavior

Well that seems dumb?

• GeoJSON spec doesn’t specify a winding order

• S2Loop (the C++ polygon implementation)

– Picks the “inside” of the polygon to be the smaller of the two regions enclosed

• When you are asking small mobile app type queries it doesn’t really matter

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Why is this even a problem?No one ever looks for a polygon like this …

I am an airplane at [0, 0]What airports are within my flight range?

I am an airplane at [0, 0]What airports are within my flight range?

I am an airplane at [0, 0]What airports are within my flight range?

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Start with a plane with a medium sized flight range polygon

I am an airplane at [0, 0]What airports are within my flight range?

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If it’s a longer range plane, that polygon gets bigger

I am an airplane at [0, 0]What airports are within my flight range?

Eventually polygon get so big it covers more than 50% of the planet

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I am an airplane at [0, 0]What airports are within my flight range?

Well, that’s a problem …Mongo 2.6 doesn’t let you query by a “Big” Polygon

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Mongo 3.0 BigPolygon

• GeoJSON shapes are defined inside a “Coordinate Reference System” (CRS)

• 3.0 has a new CRS that respects winding order

• Nothing changes unless you explicitly use the new CRS

• As of writing this, the CRS is:

"urn:x-mongodb:crs:strictwinding:EPSG:4326"

What you can do in 3.0

• Everything you used to be able to do with no code changes

• You can now specify big query polygons without ambiguity or hard to predict behavior when the polygon gets “big”

• You can query by a polygon specified in the new CRS– $geoWithin, $geoIntersects– That polygon is allowed to be BIG

• That’s it … nothing else is new

How do I do that?

// build some geo JSON

var crs = "urn:x-mongodb:crs:strictwinding:EPSG:4326”;

var bigCRS = { type : "name", properties : { name : crs } };

var bigPoly = { type : "Polygon",

coordinates : [

[[-10.0, -10.0],

[10.0, -10.0],

[10.0, 10.0],

[-10.0, 10.0],

[-10.0, -10.0]]],

crs : bigCRS

};

var cursor = db.<collection>.find({

loc : { $geoWithin : { $geometry : bigPoly } }

});

var cursor = db.<collection>.find({

loc : { $geoIntersects : { $geometry : bigPoly } }

});

This makes some developer tasks way easier

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Other people will still have to deal with the fact Geo Coordinate systems are weird

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